1. Field of the Invention
The present invention relates generally to a liquid-level meter and is directed more particularly to a liquid-lever meter in which variations of liquid level is converted into the variation of torque, which are then detected electrically.
2. Description of the Prior Art
A prior liquid level meter is normally constructed as shown in FIG. 1, in which a drum 1, a drum shaft 2 and a movable contact 5 are integrally coupled together. Slip rings are mounted on one side of a worm wheel 4 and fixed contacts 6 and 7, which are spaced from each other, are mounted on the other side in cooperation with the contact 5. The drum shaft 2 and the worm wheel 4 are coaxially mounted to be freely rotatable relative to each other. The shaft 2 and worm wheel 4 are resiliently coupled to each other by means of a spring 11 which is stretched between a spring stop 17 fixed to the worm wheel 4 and another spring stop 18 fixed to the drum shaft 2, which stop 18 also holds the movable contact 5. A pair of springs are used in practice although only one spring 11 is shown in this example for the sake of brevity. The worm wheel 4 meshes with a worm 8, which is rotatably driven by a motor 9. A wire 12 is wound on the drum 1 to suspend a displacer 10 at its free end.
When the displacer 10 is floated on a liquid surface to be measured (not shown), the tension of the spring 11 is selected so as to be balanced with the force or torque rotating the drum shaft 2 through the gravitational action on the wire 12. At this time, the movable contact 5 is arranged to be at the neutral position between the fixed contacts 6 and 7 so as not to contact with either of them, and the motor 9 is kept in a non-rotatable condition.
Now, if the liquid surface as lowered, the displacer 10 remains in the air making its buoyancy null. Accordingly, the gravitational force tending to rotate the drum shaft 2 clockwise (in a direction indicated by an arrow a.sub.1 in FIG. 1) becomes greater than the tension of the spring 11 so that the drum shaft 2 is in fact rotated rightward relative to the worm wheel 4. As a result, the movable contact 5 is brought into contact with one of the fixed contacts, namely the contact 7 in the example of FIG. 1, so that the power source AC is made to supply a current through slip rings 3 and brushes 13 through a control circuit 14 to the motor 9. Thus, the control circuit 14 is actuated to supply a current to the motor 9 turning the worm wheel 4 via the worm 8 and accordingly the drum 1 in the right direction, or in the direction shown by arrow a.sub.1. The rightward rotation of the drum 1 causes the wire 12 to move downwards to lower the displacer 10. When the displacer 10 reaches the liquid surface, the movable contact 5 is detached from the fixed contact 7 to open the above-mentioned closed circuit so that the motor 9 stops its rotation and the liquid-level meter is returned to its balanced condition as described above. On the contrary, when the liquid surface rises, the displacer 10 sinks into the liquid so that its buoyancy is increased. Therefore, the force acting on the drum shaft 2 is reversed and the movable contact 5 is brought into contact with the other fixed contact 6 and the power source AC is consequently caused to supply a reverse current through the control circuit 14 to the motor 9 to rotate it in such a direction as to lift up the displacer 10. Thereafter, when the balanced condition is again obtained, the motor 9 stops its operation.
In the prior art liquid-level meter as described above, the mechanically operable electric contacts 5 to 7 are used and the slip rings 3 and brushes 13 are also required in order to detect the variation of torque acting on the drum shaft 2, these mechanical components i.e. contacts, slip rings and brushes, are liable to wear. In addition, it is difficult for the space between the movable contact 5 and the fixed contact 6 or 7 to be made so small and hence the variation of torque acting on the drum shaft 2 can not be detected unless the torque variation are to some extent rather great. As a result, there is a drawback such that the sensibility of detecting the torque variation, and accordingly the sensibility to be required for the liquid-level meter, are limited. In FIG. 1, 15 designates a liquid level indicator provided in connection with the worm 8 through a belt 16 to indicate a level of the liquid.